Highly sensitive instrumentation for immunoassay techniques has been developed to enable measurement of reactions of extremely small quantities of biological and chemical substances. For example, instruments for radioimmunoassay are used which are sensitive, accurate and precise, but require expensive gamma-counting equipment. Other disadvantages of such systems include the short half-life of the radioisotopes, and the danger of using and disposing of the radioactive compounds used in such assays.
Another prevalent technique is the colorimetric enzyme immunoassay which utilises enzymes as labels. An enzyme-linked immunoreactant binds either to an antigen or to an antibody, causing a reaction which yields a quantitative measure of the antibody or antigen, which can be detected by a colour change. Such an assay is usually slower than other conventional techniques involving automated assays.
A third method that can be used is a fluorescence immunoassay, based on the labelling of an antigen or antibody with fluorescent probes. U.S. Pat. No. 4320970 discloses a photon-counting fluorimeter that may be used in such an assay. Disadvantages of such a system include the necessity of processing only one sample at a time. Other systems attempt to use laser beams as the external light source to excite the solution, as disclosed in U.S. Pat. No. 3984533. Again, this system can process only one sample at a time.
Instrumentation for luminescence assays advantageously involves a self-exciting luminescing system, in direct contrast to fluorimeters which utilise an external light source. In general, existing luminometers are complex in operation and require the use of substantial quantities of the reagent being sampled.
Further efforts to analyse more than one reagent sample simultaneously, in a quantitative sense, have not been successful. Efforts toward this end are illustrated by a system described by Schroeder et al in "Immunochemiluminometric Assay for Hepatitis B Surface Antigen", Clinical Chemistry, Vol. 27 No. 8 (1981), wherein a carrier is prepared containing a plurality of reagents for analysis by a luminometer which measures light production during reaction by photo-counting. However, this method and apparatus have the disadvantage of requiring the reactions to be measured sequentially, one at a time. A microprocessor was used to control fluid and air valves for adding the desired chemicals to each well; the carrier was moved in an x-y plan, to position the individual wells sequentially over a phototube, in order to enable the photons emitted by the reaction to be counted. The results were displayed by a printer. Even though photons were counted for 2 seconds at 10 second intervals, obviously a great deal of time would be required to analyse hundreds or thousands of test specimens in sequential order, one at a time.
In addition, GB-A-2132347 discloses a chemiluminometer for simultaneously handling multiple samples. The results obtained, however, are only semi-quantitative.
It has heretofore not been possible to carry out luminescent assays on multiple, small volume samples simultaneously in a short period of time, i.e. seconds. Presently existing technology permits only one such assay at a time and often requires large volume samples, i.e. of 200 .mu.l or more.
As used herein, the terms "luminescent" and "luminescence" mean all kinds of light emission except incandescence and include chemiluminescence, bioluminescence, prompt fluorescence, delayed fluorescence and phosphorescence and the like.
Rees et al, J. Phys. E: Sci. Instrum. 14 (1981) 229-233, describe a miniature imaging photon detector with a transparent photocathode. It is proposed for use in astronomy and geophysics.